[SI-LIST] Re: 90 degree bend
- From: SreejaPillai@xxxxxxxxxxxxxx
- To: pcb_user@xxxxxxxxxxx
- Date: Mon, 6 Sep 2004 18:10:59 +0530
Hi New Bee .......nice name ...make sure u change it after few years ;-=
)
...
I agree to Scott & Craig... Well I guess we are all passing on the same=
info in our own ways right...
This is what I think about 90deg bends..
Abrupt signal endings ie. discontinuities changes the signal impedance
abruptly(Discontinuities can be sharp bends on the trace or at vias thr=
ough
board.)
At bends on the trace, the cross-sectional area increases,and Z0 decrea=
ses.
It is possible to compensate for the bend by cutting the trace.The cut =
is
chosen so that the resulting diagonal is equal to the trace width. This=
minimizes the delta in cross-sectional area, as well as the
discontinuity.The vertical run of metal between layers is an uncontroll=
ed
impedance, and the more of these there are, the greater is the overall
amount of uncontrolled impedance in the line. This contributes to
reflections. Also, the 90=B0 bend from horizontal to vertical is a
discontinuity that generates reflections. If vias cannot be avoided,use=
as
few as possible.
As Scott said, we could account for a 1 or 2 pico seconds delay in our
board, well explained by Scott.
There doesnt seem to be any signal integrity issues in time domain.The
impedance changes are not measurable even on a 3GHz BW N/W analyzer. Th=
e
velocity of propagation of the signal is oblivious to the discontinuity=
unless the signals are in upper GHz freq and the edge rates are faster =
than
10ps - 15 ps.
Same is the case with frequency domain - not much emissions are measure=
d in
frequency domain also. The discontinuities within component packages ,
connector pin-outs, layer jumping of routed traces, vias etc in
transmission lines will radiate at levels that far exceeds the ones due=
to
90 degree bends.
(Steve.. Pls correct me if I am wrong!!!)
Regards,
Sreeja S Pillai
Engineer - Hardware
SCM Microsystems ( India ) Pvt. Ltd.
=
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